The present invention relates to a method for the formation of superporous hydrogels having improved physical and mechanical properties.
superporous hydrogels (SPH) are chemically crosslinked hydrophilic polymers that contain a multiplicity of pores with diameters in the micrometer to millimeter range, enabling them to absorb tens of times their weight of aqueous fluids i just a fraction of a minute. SPH pores are interconnected in the hydrogel matrix such that absorbing fluid can move freely through the channels (capillaries), allowing them to swell much faster than conventional hydrogels that have the same swelling capacity.
To prepare a superporous hydrogel, a monomer, a crosslinker, a solvent (normally water), a surfactant (for foam stabilization), and a foaming aid are first mixed together, followed by the addition of an initiator. A blowing agent is then added to the mixture for the generation of gas bubbles such as, for example, the generation of caron dioxide. Once the initiator and blowing agent are added, foaming and polymerization (also refer as gelling) processes take place simultaneously. As polymerization proceeds, the viscosity of the reaction mixture increases and the bubbles being generated are trapped within the highly viscous polymer matrix. The foaming resulting from simultaneous gelation and bubble formation continues until both processed are stopped. At this stage, the product takes the form of flexible foam. To remove residual monomer, non-reacted crosslinker, and initiator impurities, the flexible foam is washed thoroughly with water. After this purification process, a water miscible alcohol such as, for example, ethanol, is added and subsequently removed to dehydrate the hydrogel. A final drying step is usually performed in an oven, preferably in a vacuum oven at low temperatures. U.S. Pat. No. 6,271,278 describes the preparation of various SPHs in detail. SPHs are also described by Chen, et al., in J Biomed. Mater. Res. 44:53–62 (1999).
Superporous hydrogels are generally prepared based on hydrophilic monomers, including acrylic acid and its salts, acrylamide, the potassium salt of sulfopropyl acrylate, hydroxyethyl acrylate, and hydroxyethyl methacrylate. A desirable superporous hydrogel would possess high solvent absorption properties yet be able to withstand external forces such as tension, compression, and bending. Some hydrogels have been prepared that have desirable swelling capacity and swelling rate properties. However, very high swelling superporous hydrogels are normally very loose after swelling and, when a small amount of pressure is applied, easily break apart. Some hydrogels have also been prepared that have reasonable mechanical properties, such as strength, ruggedness, and resiliency. However, very strong superporous hydrogels absorb limited amounts of water and thus have undesirable swelling properties. A preparation method for producing a hydrogel with both adequate swelling and mechanical properties is still lacking in the art.